/Engine/Source/Runtime/RenderCore/Public/ShaderCore.h
If a compile argument with the given name exists and is of the specified type, its value will be assigned to OutValue and the function will return true. Otherwise, either the named argument doesn't exist or the type does not match, the OutValue will be left unmodified and the function will return false.
template<typename ValueType> bool GetCompileArgument ( const TCHAR * Name, ValueType & OutValue ) const